In sentimental moments, I’m tempted to say my brother’s birth is my first memory because it was the first thing in my life worth remembering. There could be a sliver of truth to that: Research into the formation and retention of our earliest memories suggests that people’s memories often begin with significant personal events, and the birth of a sibling is a textbook example. But it was also good timing. Most people’s first memories date to when they were about 3.5 years old, and that was my age, almost to the day, when my brother was born.